PFC test theory question

[zakjay]

Hi all! I'm at the start of a long road to become a sparky's mate, and I'm tryina wrap my head around something that might seem really basic. It's to do with the prospective fault current test. Let's say I get a reading of 1.2ka, and the main cutout fuse is BS 1361 type 1 which is 16.5ka. This means that if something goes wrong the cutout fuse can handle it and break the circuit.

My question is how does a cut out fuse know when to cut the circuit? How does it know when it's a short circuit or earth fault and when it is not?

 

[fairlight]

Hi all! I'm at the start of a long road to become a sparky's mate, and I'm tryina wrap my head around something that might seem really basic. It's to do with the prospective fault current test. Let's say I get a reading of 1.2ka, and the main cutout fuse is BS 1361 type 1 which is 16.5ka. This means that if something goes wrong the cutout fuse can handle it and break the circuit.

My question is how does a cut out fuse know when to cut the circuit? How does it know when it's a short circuit or earth fault and when it is not?

The cutout fuse is the 'strongest link' in a chain of weak links. Final circuits will have OCPD's at their origin that should break the circuit before fault currents reach a sufficient level to operate the head fuse. If they should fail the cutout fuse is there to break fault current before any major damage is done. Look at app3 in the BBB. Time/current characteristics show how different devices will operate at various fault/overload currents. The fuse cannot tell whether a current is a short circuit or earth fault current, it will simply operate when the current is at a certain level for a specific duration.

 

[zakjay]

Thanks for taking the time to explain

Thanks for the guidance on where to look in BBB. it says that BS 88-3 replaced BS 1361, I had a look any way at tables for BS 88-3. There's one for 0.4 s and 5 s disconnection times for max earth fault loop impedance. I'm not sure these tables help. Aren't they just showing me the max permissible resistance allowed for the fuse to operate within the right time?
If I use the BBB table:
For a 63 amp fuse it's 0.68 ohms
My reading was 1.2ka so if I use V=IR I get 0.2 ohms

But all this is proving is I'm withing the allowed operating resistance,

I guess the point of a pfc test is just to make sure the fault current doesn't exceed the operating current of the cut out fuse, but why would this matter anyway? Since it would blow the fuse if it's anything over and open the circuit. Thereby doing what it was supposed to.

Also there is always a current flowing back to the cutout so I guess at some minimum threshold the fuse blows. It needs to know that minimum is a fault and not just normal operating current.

Sorry if these are basic questions :S

 

[DPG]

Bear in mind the current that the fuse will open at is not the same as the breaking capacity. The breaking capacity is the absolute maximum current that it can safely interrupt without exploding /catching fire etc

 

[James]

Fuses and circuit breakers all have a rated capacity and a maximum fault current.
When a fuse blows, the wire melts and creates a gap between the 2 fuse terminals.

Let’s say a 63A fuse has 90A running through it, after a few seconds it will blow, by melting the fuse wire and the current flow will stop.

Now imagine a dead short that puts 10,000A through it.
The wire will melt almost instantly but in doing so, may be replaced with an arc, where the current is flowing through the air inside the fuse.

That’s why fuses are different physical sizes and have different materials inside them to extinguish the arc and ensure the current flow is stopped.

A similar thing can happen to circuit breakers.

 

[Deleted member 105166]

Bear in mind the current that the fuse will open at is not the same as the breaking capacity. The breaking capacity is the absolute maximum current that it can safely interrupt without exploding /catching fire etc

Indeed the latter value being hundreds of times the former, for example a 6A Type B MCB will open between 18A and 30A but the physical device is rated to sustain fault currents up to 6,000A in the case of a 6kA variant.

 

[fairlight]

Thanks for taking the time to explain

Thanks for the guidance on where to look in BBB. it says that BS 88-3 replaced BS 1361, I had a look any way at tables for BS 88-3. There's one for 0.4 s and 5 s disconnection times for max earth fault loop impedance. I'm not sure these tables help. Aren't they just showing me the max permissible resistance allowed for the fuse to operate within the right time?
If I use the BBB table:
For a 63 amp fuse it's 0.68 ohms
My reading was 1.2ka so if I use V=IR I get 0.2 ohms


But all this is proving is I'm withing the allowed operating resistance,

I guess the point of a pfc test is just to make sure the fault current doesn't exceed the operating current of the cut out fuse, but why would this matter anyway? Since it would blow the fuse if it's anything over and open the circuit. Thereby doing what it was supposed to.

Also there is always a current flowing back to the cutout so I guess at some minimum threshold the fuse blows. It needs to know that minimum is a fault and not just normal operating current.

Sorry if these are basic questions :S

The maximum permissible Zs determines the minimum current required to operate the fuse in the required time in the event of a fault of minimum impedance to earth. Remember though that a short circuit current is between live conductors and may be considerably higher than an earth fault current, particularly on a 3 phase supply. The device at the origin must be capable of interrupting the highest fault current, usually a short circuit current. That's why the PSCC and PEFC are both measured, as long as both are below the breaking capacity of the fuse all is good.
In domestic and small commercial it's unusual to get a PFC anywhere near the breaking capacity of most devices. Readings between 1Ka to 4Ka are most common.

 

[Deleted member 105166]

In domestic and small commercial it's unusual to get a PFC anywhere near the breaking capacity of most devices. Readings between 1Ka to 4Ka are most common.

...unless you're close to the substation...

 

[James]

...unless you're close to the substation...

I was going to say that, I did a job recently in a plant room containing the transformer.
testing just came up with silly numbers.
a call to the transformer manufacturer put me right!!
its all about resistance and inductance, but that goes a little beyond this discussion.

 

[zakjay]

Thanks for your input guys, it makes a lot of sense now why I need to check that I don't go over the 16.5ka cut out fuse. Because if it did, it could cause melting and fire etc. And so I guess this is why we do a PFC test.

I'm having trouble with the other end of things. What I mean is, current is always flowing back to the cutout fuse via the neutral tail right? So am I right in thinking that if 100a flows up the neutral tail into the cutout the fuse will break? And the reason a main fuse is 100 amps is because it's unlikely a standard domestic CB will use this much if you include diversity?

 

[Marvo]

According to Kirchoff's Law current flowing into a circuit will equal current flowing out so if there's a 100Amp load on a CU there will be 100 Amps flowing into it via the live tail and 100 Amps flowing out via the neutral tail assuming there's no current leakage to earth.

The reason a main fuse is 100Amps is to protect the suply wiring from overload.

Fuses and circuit breakers are sized according to the capability (ampacity) of the wiring in the circuit they supply.

 

[M-Ty]

Zakjay,
Please have another look at your at tables for BS 88-3 and some time versus current graphs (curves) for them and you'll see that at 100 amps (r.m.s.) through the fuse (which by definition implies it doesn't matter which direction or if it varies at 50Hz; probably worth looking the definition (of rms) up for a proper explanation) it will not "break." The current only flows for 20 milliseconds "up" the neutral tail into the cutout, reaching its peak sinusoidally at 10 milliseconds, before reversing direction and going "down" the cutout and neutral tail.
(As Marvo says, assuming there's no current leakage to earth, otherwise it's a slightly different bag of worms.)

 

[davesparks]

My question is how does a cut out fuse know when to cut the circuit? How does it know when it's a short circuit or earth fault and when it is not?

The fuse in the cutout is the same as any other HRC fuse, it doesn't 'know' the reason for the current flowing through it, it just responds in the way it has been designed to work.
A fuse is a piece of wire which has been made to a precise set of dimensions. This is designed so that it behaves in a reliable and predictable way when current flows through it.
If it is a 100A fuse then if 100A or less flows through it it will get a little warm but nothing will happen.
If more than 100A flows through the fuse then it will start to get hotter and eventually reach a point where the fuse breaks.
If a small overload occurs, for example 150A flows then it will take a relatively long time before the fuse breaks.
Wheras if a lot of current flows, say 1000A it will break a lot quicker.

There are graphs in the regulations which show how the speed of the fuse breaking relates to the size of the current flowing through it.
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I'm having trouble with the other end of things. What I mean is, current is always flowing back to the cutout fuse via the neutral tail right? So am I right in thinking that if 100a flows up the neutral tail into the cutout the fuse will break? And the reason a main fuse is 100 amps is because it's unlikely a standard domestic CB will use this much if you include diversity?

The current in the neutral doesn't flow through the fuse, fuses are only ever installed in the line conductors (in a normal installation)
Assuming we are discussing a single phase supply you will have one fuse in the cutout which is in the line, the neutral is a solid connection block.

A fuse wont break immediately if the current exceeds it's rating, but the higher the overload the faster it breaks.
If you have a 100A fuse and 101A flows through it then it won't break at all. If 110A flows through a 100A fuse then it could take days for it to break.

 

[DPG]

Well described Dave!

 

[zakjay]

thanks a lot everyone for taking the time to explain. You made it very easy to understand. I took a look at the graph in BBB. I understand now why we do a PFC test and why a fuse allows a current until it starts reaching over 100 amps. But there's one thing I don't understand, what if your normal design current is supposed to be over 100 amps? I know it's unlikely but say you have 2 x 10 kw showers, electric hob, etc and other things. I guess this is where diversity comes into play and also the cut off time of the fuse on the graph. But worst case scenario, shouldn't there be fuse greater than 100 amp available? Or is that when you start considering a 3 phase supply?

 

[fairlight]

thanks a lot everyone for taking the time to explain. You made it very easy to understand. I took a look at the graph in BBB. I understand now why we do a PFC test and why a fuse allows a current until it starts reaching over 100 amps. But there's one thing I don't understand, what if your normal design current is supposed to be over 100 amps? I know it's unlikely but say you have 2 x 10 kw showers, electric hob, etc and other things. I guess this is where diversity comes into play and also the cut off time of the fuse on the graph. But worst case scenario, shouldn't there be fuse greater than 100 amp available? Or is that when you start considering a 3 phase supply?

In a 'normal' domestic installation line current rarely exceeds 100a, even with showers/ovens/electric heating etc it is unlikely that all will be in use at the same time, that is the principle of diversity. Many dwellings run without issues on 60a cutout fuses. Where line current is expected to exceed 100a then 3 phase supplies are used with the load balanced as far as possible over the 3 phases.

 

[M-Ty]

I made a right pig's ear of describing a 50Hz sinewave...

 

[zakjay]

Got it, thanks a lot, I appreciate everyone passing on their knowledge, I have a much better understanding, still a long way to go